1. Field of the Disclosure
Embodiments of the present disclosure relate generally to a gas piston system for a firearm.
2. Description of the Related Art
FIGS. 1A and 1B illustrate a prior art M16 rifle. The M16 rifle includes an upper receiver 50 hinged to a lower receiver 51 at a pivot pin 52. A removable pin 53 extends through the lower receiver to hold the upper receiver 50 in place. A charging handle 54 at the rear of the upper receiver is provided for charging the weapon. Automatic and semi-automatic operation of the weapon is achieved by a gas tube 56 extending from a forward portion of the barrel 57 to the receiver. A removable ammunition magazine 58 is inserted in the lower receiver.
The upper receiver 50 has a hinge aperture 65 for receiving the hinge pin 52, and a downwardly extending boss 66 having an aperture 67 for receiving the retaining pin 53. The gas tube 56 extends through the front of the upper receiver 50 and enters a gas chamber 68 for affecting the backward movement of a bolt carrier 55 upon the firing of a cartridge 76. The bolt carrier 55, upon firing, moves backwardly into the gunstock 69 against the action of a recoil spring 70 and buffer 71 in the stock. The charging handle 54 slidably extends into the upper receiver, and carries a projection 74 which engages a projection 72 on the bolt carrier 55 upon rearward movement of the charging handle 54, to affect the manual charging of the weapon. The charging handle 54 also has an elongated internal slot 73 for clearance of the bolt carrier 55 during operation of the weapon. An automatic sear 75 of conventional nature is provided in the lower receiver according to the conventional practice. The sear 75 is operated by the bolt carrier 55, for catching an upper hook 78 of the weapons hammer 77 during automatic operation of the weapon. In semi-automatic operation the trigger mechanism (not shown) of the weapon catches the lower hook 79 of the hammer. The cartridge 76 is in firing position in the firing chamber of barrel 57. The magazine 58 is held in the lower receiver so that cartridges are fed from the top of the magazine to the bolt upon forward movement of the bolt carrier 55.
The standard design gas system used in AR15 and M16 rifles and M4 carbines utilizes a direct gas impingement (DGI) system which directs expanding gas from the fired cartridge out of the barrel 57 through a gas port in the barrel. The expanding tapped gas is then directed through the gas tube 56 which directs the gas back into the upper receiver. The gas then enters the bolt carrier key forcing the bolt carrier 55 to the rear and unlocking the bolt, beginning the cycling process.
All gas piston systems operate in much the same way; they use propellant gases from the fired cartridge to actuate a piston, which pushes on a rod that cycles the weapon. Most gas piston systems currently available for the AR15 weapon system are retrofit systems made to convert the existing DGI equipped rifles and carbines to a piston system. These piston systems use the existing gas port location and gas port diameter already in place on the DGI configured weapons, making them desirable to owners of these commonly configured weapons.
Most all of these retrofit gas piston systems are also designed to operate with the most common cartridge found in the AR15 weapon platform, the 223 Remington (civilian designation) or the nearly identical 5.56×45 millimeter NATO (military designation) used in the M16 rifle and M4 carbine. These retrofit systems are able to work with existing gas port sizes and locations common to this weapon system mainly because the standard chambering mentioned above has enough “gas port pressure and volume” to activate the piston system. With any of the standard length systems; carbine length, mid length, or rifle length, a piston system generally requires more gas volume and pressure to operate than a DGI system.
The front end of the rifle and/or carbine, often referred to as the “hand guards”, is standardized in three different lengths to coincide with the three gas system lengths found on DGI equipped guns. The gas block attached to the barrel where the gas is “tapped” from the barrel is located just in front of the hand guards, this is also where the gas blocks are for most piston systems. Because the gas blocks are out in front of the hand guards on the barrel the size of the components can be adjusted or enlarged to give the desired performance. This is also the location for the exhaust port on all piston systems, where the hot and dirty propellant gases are discharged.
The fact that piston systems require more gas port pressure and volume to operate, and that most of them use the existing gas port locations and diameters means that they may not function reliably with all available brands and types of ammunition. This is because ammunition manufactures use many different types of propellants in their ammunition to obtain the best performance with the many different bullets weights and styles that are available.
Each propellant has its own burning characteristic and develops its own “pressure curve”. The pressure curve in basic terms is the time it takes a specific propellant to reach its maximum pressure and how fast that pressure drops off as the bullets moves down the barrel; the charted profile of a propellant igniting, its build up of pressure, its maximum pressure, and drop in pressure is the pressure curve. Most gas ports in the barrels are located on the “down slope” of the pressure curve, if a given propellant is too far down its down slope by the time the bullet reaches the gas port the weapon will not have enough port pressure or volume to cycle the weapon.
With few exceptions, gas piston systems for the AR15 rifle work as long as the standard caliber (223 Remington/5.56-mm NATO) for this weapon system is used; in most loads this cartridge provides ample port pressure and volume to operate either system. If cycling or functioning problems occur with certain types of ammunition, then the piston components can be enlarged to give the system more force to operate the weapon and increase reliability because there is little size constraint out in front of the hand guards. Existing gas piston systems currently available for the AR15 rifle are adequate because most of these rifles are chambered for the standard cartridge mentioned above.